The papermaking industry continues to be interested in alternative ways to enhance the wet strength of paper products. The continued commercial importance of paper products such as carrier paperboard, tissue and towel drives the quest for improved compositions and methods to enhance the wet strength of paper products.
Polyamidoamide-epichlorohydrin (PAE) resins are commonly used as permanent wet strength agents for manufacturing wet strength paper grades. Typically, the wet strengthened towel grades require high dosage levels of PAE resin to achieve the required wet tensile specifications. The amount of the PAE resin that can be adsorbed onto cellulose fibers is limited by the anionic charge density of the fibers. If not properly managed, unretained wet strength resins will accumulate in the white water system leading to poor machine dewatering, wire and felt filling, sheet breaks and holes, and increased defoamer usage. To overcome these unwanted effects, the system charge is often balanced by applying anionic chemicals such as carboxymethyl cellulose (CMC) and/or anionic synthetic resins.
Carboxymethyl cellulose (CMC) is widely used in production of wet strengthened towel. CMC is reasonably inexpensive when supplied in dry form, either powder or granules. This form requires a makedown system for dissolution prior to use. CMC is prone to biological growth. Another drawback of CMC can be decreased dewatering of the fiber suspension. Both CMC adsorbed to the fiber surfaces and CMC in the liquid phase cause deflocculation of fiber suspension and an increase of the filtration resistance. Thus, the use of CMC can increase the demand for retention aids on paper machines.
Synthetic dry strength resins are often based on polymerization of acrylamide and acrylic acid monomers. The acrylamide-acrylic acid copolymers can be manufactured within a wide range of molecular weight and anionic charge. For instance, these polymers are available as solutions having active polymer solids from 18 to 25%. Solution polymer molecular weight ranges are limited, generally less than 500,000 Dalton, because the bulk viscosity must be low enough to allow the product to be pumpable.
Polyacrylamide dry powder products typically have molecular weight (MW) in the range 10-15 million Dalton. They are cost efficient and easily delivered to remote or oversea customer sites. They are widely used in the treatment of municipal and industrial wastewater.
However, the use of polyacrylamide dry powder products in paper production is not that straightforward. Dry powder products cannot be used as paper dry strength agents because they have a negative effect of overflocculating the sheet due to the extremely high molecular weight these dry polymers carry.
Polyacrylamide products used as paper dry strength agents typically have molecular weight in the range of from 300 000 to 500 000 Dalton. Conventional dry strength agents comprising polyacrylamides are delivered as aqueous solutions that can be further diluted with non-specialized equipment. These conventional dry or wet strength agent solutions usually have about 20% by weight of delivery solids and their target molecular weight is less than 500 000 Dalton due to the bulk viscosity limit for pumping in paper mill applications.
An example of a functional promoter and dry strength resin currently sold to tissue mills is an anionic polyacrylamide product solution with 20% by weight of solids. This aqueous dry strength resin solution is spray dried to produce suitable dispersed powders for those customers who prefer a delivery in dry form. However, a spray drying process is not cost efficient, and significantly increases the manufacturing costs.
There is a need to provide a functional promoter and a dry and/or wet strength agent in a dry powder form which can be efficiently manufactured, transported and regenerated into applicable form suitable for use in a pulp suspension at a paper mill production line.